Multibeam Sonar Theory of Operation

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Multibeam Sonar Theory of Operation Sidescan Sonar To maximize the sidescan capabilities of the SEA BEAM 2100, you need to understand the processes used to collect it. This section gives a brief overview of the steps of sidescan data collection performed by the sonar, with particular emphasis on where and when sidescan returns can be influenced by user intervention. Producing Sidescan Data With the SEA BEAM 2100 The SEA BEAM 2100 sidescan sonar uses the same hardware as the depth-sounding sonar. In fact, it uses the same pings and much of the same processing. Only fairly late in the data processing do the procedures for measuring bathymetry and generating sidescan imagery diverge. This arrangement allows the SEA BEAM 2100 to produce both types of data without much more computation than would be required for each individually. As described in previous chapters, the projectors in the SEA BEAM 2100 system are arranged in a line array, similar to a traditional sidescan sonar. The projector array is aligned with the direction of travel of the survey vessel, although rather than being towed, it is mounted on the ship’s hull. This array ensonifies a strip of the bottom perpendicular to the survey vessel’s track. Instead of using line arrays for both transmission and reception in traditional sidescan sonar, the SEA BEAM 2100 has an array of hydrophones (between 48 and 80, depending on the installation), mounted athwartship— at right angles to the projector array. The data from the hydrophones is processed to extract both amplitude and angle information. While this processing is a complex procedure, it can be understood as a series of fairly straight-forward steps. Many of the steps in the procedure are the same as those required for bathymetry measurement. Specifically, the processing required for Beam Forming and Beam Steering, High-Resolution Angle Estimation and Adjusting Final Angles for Roll is identical for both types of measurements. These processes are all described in detail in earlier chapters (Beam Forming and Beam Steering in Chapter 3, and Angle Estimation and Roll Compensation in Chapter 4). To briefly review: • Beam Forming and Beam Steering – The amplitude and phase information from all of the elements in the hydrophone array are passed through an FFT to obtain amplitudes detected by several steered beams (the number of beams depends on the installation). • High-Resolution Angle Estimation – The measurements of all beams in each time interval are collected into a time slice. Within each time slice, the response pattern of the beam is used to precisely determine the angle to specific echo events that occur in that time slice. • Adjusting Final Angle for Roll – The angles measured in the above item are adjusted to the changing angle of the hydrophone array, yielding the true angles to echoes on the sea floor. BDI bathymetry and sidescan processing essentially share the result of these three procedures. Emerging from them is a list of time slices, each with amplitudes and direction angles for a number of echo events (called hits). At this point, the Bathymetry and Sidescan processing diverge. Copyright © 2000 L-3 Communications SeaBeam Instruments Page 5-13 No portion of this document may be reproduced without the expressed written permission of L-3 Communications SeaBeam Instruments
Sidescan Sonar Multibeam Sonar Theory of Operation Mapping Sidescan Values for 2000 Pixel Resolution The final stage of sidescan data processing is mapping the detected echoes to build an image of the sea floor. In detecting the echoes of a specific ping, the SEA BEAM 2100 monitors the particular set of sequential time slices that generally range between when the first (nearest) and last (farthest) bottom echoes are expected. This range is controlled by the SEA BEAM 2100 operator through the Start and Stop Gates settings (see Chapter 9 of the SEA BEAM 2100 Operator’s Manual for information about the theory and operation of the Start and Stop Gates settings). Recall that the sidescan process collects the echoes from a swath of the sea floor that is oriented perpendicular to the path of a survey vessel. For each bottom swath, the SEA BEAM 2100 sidescan data is mapped into an array of 2000 pixel locations. This array contains the amplitudes of echoes from the sea floor. The physical size represented by these 2000 pixels is determined by the value of sidescan pixel size, which is set by the SEA BEAM 2100 operator (see Chapter 10 of the SEA BEAM 2100 Operator’s Manual for information on setting sidescan pixel size). For instance, if the sidescan pixel size is set to 2 meters per pixel, the 2000-pixel sidescan swath is 4000 meters wide. In all cases the mapped swath is centered directly below the path of the survey vessel. The echoes from all relevant time slices are added to the sidescan swath array. The echo angles are converted to physical locations on the bottom based on sound velocity. These are then mapped into the pixels in the sidescan swath array (see Figure Chapter 5 - -16). Amplitudes of echoes that fall within the physical limits of the 2000 pixel swath are retained; all others are discarded. At the conclusion of the process, some pixels will have no echoes recorded in them, some will have one, and others will have many. Pixels that have one echo are assigned the amplitude of that echo. If a pixel has more than one echo, the average of the amplitudes is used. Pixels with no reported echoes are assigned a value interpolated from neighboring pixels. Page 5-14 Copyright © 2000 L-3 Communications SeaBeam Instruments No portion of this document may be reproduced without the expressed written permission of L-3 Communications SeaBeam Instruments
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